Since an aluminum alloy exhibits a lightweight nature and a high thermal conductivity, and since a high corrosion-resistant feature can be realized in the aluminum alloy by a suitable treatment, it is utilized in a heat exchanger for an automobile, for example, a radiator, a condenser, an evaporator, a heater, an intercooler and so forth. For a tube member for the automobile heat exchanger, a 2-ply clad material or a 3-play clad material is used: the 2-ply clad material includes a core material composed of an Al/Mn-based alloy such as JIS3003 alloy and so forth, and an Al/Si-based filler material or an Al/Zn-based sacrificial anode material cladded on one surface of the core material; or the 3-ply clad material includes a core material composed of an Al/Mn-based alloy such as JIS3003 alloy and so forth, an Al—Si-based filler material or an Al/Zn-based sacrificial anode material cladded on one surface of the core material, and an Al/Si-based filler material cladded on the other surface of the core material. Usually, one of these clad materials is combined with and joined to a corrugated fin by carrying out a brazing process at a high temperature of about 600° C. When an inner fin is joined to an interior surface of the tube member formed of the clad material, it is necessary to give a brazing property to both the interior and exterior surfaces of the tube member.
If there is a corrosive liquid within the tube member of the heat exchanger, the tube member may be perforated due to occurrence of pitting corrosion, or a wall thickness of the tube member may be reduced due to occurrence of a uniform corrosion, so that a pressure strength of the tube member declines, resulting in breakage of the tube member. As a result, an air, an cooling water or a cooling medium circulated in the interior may be leaked. Conventionally, a corrosive liquid in the interior of the tube member is a neutral liquid or a weak acid liquid, and, in such a corrosive environment, in order to obtain both an external brazing function and a corrosion resistance, the heat exchanger is made of a brazing sheet on which a Zn-added and Al/Si-based filler material is cladded.
Incidentally, recently, in order to comply with an exhaust gas regulation in EU and USA, a system in which an exhaust gas passes through the heat exchanger has been developed to aim at improvement of mileage. When an exhaust gas passes through the heat exchanger, an interior of the tube member is cooled due to the heat exchanging so that a condensate water containing components of the exhaust gas (which is referred to as an exhaust gas condensate water hereinafter) is produced. This exhaust gas condensate water may exhibit a strong acidity, and thus the conventional brazing sheet, on which the Zn-added and Al/Si-based filler material, is insufficient in a corrosion resistance.
A corrosion rate of aluminum is considerably subjected to an influence of pH, and the lower pH, the larger the corrosion rate. According to the potential-pH diagram, when pH is less than 4, Al3+ is stable, and thus there is general recognition that it is difficult to utilize aluminum as a corrosion-resistant material. Also, when a surface of the brazing sheet is defined by the Zn-added and Al/Si-based filler material, the corrosion rate is further increased because the Si particles on the surface of the brazing sheet serve as cathodes and because resolution of Al is accelerated due to Zn. In addition, when chloride ions exist in the solution, the passive films are broken by the chloride ions so that pitting corrosion occur therein. Since the exhaust gas condensate water contains chloride ions, it has a nature which causes the pitting corrosion. Namely, when the exhaust gas passes through the interior of the tube member of the heat exchanger, the corrosion deriving from the lowness of pH must be restrained while it is necessary to suppress occurrence and development of the pitting corrosion by giving a sacrificial protection feature to the tube member. Although the exhaust gas condensate water contains different components under various conditions, it may exhibit a strong acidity of less than pH 3, and a density of chloride ions may be more than 5 ppm which causes the pitting corrosion.
In order to solve an problem that a brazing sheet for a tube member must have a higher corrosion resistance than that of the brazing sheet on which the conventional Zn-added and Al/Si-based filler material is cladded, while giving a brazing function to both the surfaces of the tube member, Patent Documents 1-3 disclose a brazing sheet including a core material, and a material having a larger addition amount of 1.5-6.0% Si than that of the conventional case and cladded as a sacrificial anode material on the core material. Like this, when the large amount of Si is added to the sacrificial anode material, a part of the sacrificial anode material is molten during a heating process for brazing so that the molten material can serve as a filler material to braze a bare fin, and the other part of the sacrificial anode material remains as a solid part so that it is possible to obtain a higher corrosion resistance than that of the conventional Zn-added and Al/Si-based filler material. Nevertheless, it is presumed that the brazing sheets based on these techniques are used in a corrosive environment in the interior of the tube member through which a neutral liquid or a weak acid water flows like a radiator, a condenser, an evaporator and so forth, and thus a corrosive resistance of these brazing sheets is insufficient in the corrosive environment in which a liquid featuring a very low pH such as the exhaust gas condensate water exists.
In particular, in the technique disclosed in Patent Document 1, although a brazing between a sacrificial anode material and an inner fin is made possible, an addition amount of Zn is only at most 7%, and no reference is made to an idea of limitation of this addition amount. As already stated, since Zn accelerates resolution of Al, it is necessary to severely limit the addition amount of Zn before the corrosion rate can be restrained. Nevertheless, this problem is not at all recognized, and there are no reference to and no suggestion of solution of the problem that the tube member is exposed to the liquid featuring the very low pH such as the exhaust gas condensate water.
On the other hand, in the technique disclosed in Patent Document 2, in order to improve a brazing property of a surface of the sacrificial anode material, it contains Si particles having a size of 0.1-1.0 μm at a number density of 15,000-40,000/mm2. As already stated, the Si particles serve as the cathode in a corrosion reaction so that the corrosion rate is increased. The Si particles having the size of 0.1-1.0 μm are molten during a heating process for brazing so that the molten Si particles serve as the filler material, and thus the corrosion rate of the not molten remaining sacrificial anode material cannot be increased. Nevertheless, the Si particles having a size of more than 1.0 μm still exist in the remaining sacrificial anode material after the heating process for brazing, resulting in increase of the corrosion rate. Therefore, it is necessary to limit the number density of Si particles having the size of 1.0 μm, but this matter is not at all considered in the technique of Patent Document 2. Further, no reference is made to a manufacturing method for obtaining a number density of Si particles having a given size.
In Patent Document 3, although a size of Si particles and a number density of Si particles are described in detail, these descriptions are merely directed to a distribution of fine particles, and there are no reference to and no suggestion of a limitation of a number density of large Si particles having the size of 1.0 μm.